On account of the limited resources of fossil fuels, in particular on account of the limited availability of mineral oil as a raw substance for the extraction of fuels for internal combustion engines, constant efforts are being made in the development of internal combustion engines to minimize fuel consumption, wherein the focus of the efforts is on obtaining more effective combustion.
A problem is fuel consumption in particular in applied-ignition engines. The reason for this lies in the basic operating process of the traditional spark-ignition engine, which operates with a homogeneous fuel/air mixture which is prepared by means of external mixture formation by virtue of fuel being injected into the intake tract, and in which the desired power is adjusted by means of quantity regulation.
Load control is generally carried out by means of a throttle flap provided in the intake tract. By adjusting the throttle flap, the pressure of the inducted air downstream of the throttle flap can be reduced to a greater or lesser extent. The further the throttle flap is closed, that is to say the more said throttle flap blocks the intake tract, the higher the pressure loss of the inducted air across the throttle flap, and the lower the pressure of the inducted air downstream of the throttle flap and upstream of the inlet into the combustion chamber. For a constant combustion chamber volume, it is possible in this way to adjust the air mass, that is to say the quantity, by means of the pressure of the inducted air. Quantity regulation by means of a throttle flap has thermodynamic disadvantages in particular in the part-load range. This is because low loads require a large pressure drop in the intake tract, and therefore a high degree of throttling. To reduce throttling losses, various strategies have been developed, for example the dethrottling of the spark-ignition engine operating process through the use of a variable valve drive.
Internal combustion engines may be supercharged. Supercharging is primarily a method for increasing power, in which the air for the combustion process in the engine is compressed, but is generally a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. For the same vehicle boundary conditions, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. This is also referred to as downsizing.
Supercharging consequently assists in the constant efforts to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
For supercharging, use may be made of at least one exhaust-gas turbocharger in which a compressor and a turbine are mounted on the same shaft, wherein the turbine is arranged in the exhaust-gas discharge system and the compressor is arranged in the intake system of the internal combustion engine.
The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. Furthermore, a charge-air cooler may be provided by means of which the compressed charge air is cooled before it enters the combustion chamber. The charge-air cooler lowers the air temperature and thereby increases the density of the air, as a result of which the cooler also contributes to improved charging of the combustion chamber with air, that is to say to a greater air mass.
One advantage of the exhaust-gas turbocharger for example in relation to conventional mechanical chargers is that no mechanical connection is utilized between the charger and internal combustion engine.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges. According to some systems, a severe torque drop is however observed in the event of a certain rotational speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. In the case of a spark-ignition engine, for example, if the load is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. This has the result that, toward lower rotational speeds, the charge pressure ratio likewise decreases, which equates to a torque drop.
In practice, the described relationships often lead to the use of a small exhaust-gas turbocharger, that is to say an exhaust-gas turbocharger with a small turbine cross section, or of a plurality of exhaust-gas turbochargers.
In the optimization of the exhaust-gas turbocharger arrangement, the response behavior of the exhaust-gas turbocharger arrangement or of the internal combustion engine supercharged by means of the exhaust-gas turbocharger arrangement is likewise of particular interest. It is a problem here that, in certain situations, the internal combustion engine can follow up on the power demand of the driver only with a delay. In particular in the event of a so-called load step, in which the load demand rises abruptly, the exhaust-gas turbocharger reacts with a time offset to provide, that is to say build up, the charge pressure for the demanded load.
An example of such a load step is an overtaking maneuver, in which the power demand is increased abruptly and considerably, generally by several times, as a result of a kickdown of the accelerator pedal. Proceeding from operation of the internal combustion engine at part load, in which the ignition time αZ,opt is set with regard to the highest possible efficiency and therefore the ignition takes place relatively early, upon the initiation of a load step, the ignition time αZ is shifted abruptly in the late direction, that is to say follows up the abruptly increasing load. Here, the shift of the ignition time in the late direction with increasing load is basically utilized in order to avoid knocking combustion. It is nevertheless sought to perform the ignition as early as possible in order to realize as high an efficiency as possible, that is to say as early as is permitted while still avoiding knocking combustion.
The inventors herein have recognized the above issues and provide an approach to at least partly address them. Accordingly, a method for operating a supercharged internal combustion engine including at least one exhaust-gas turbocharger which has a turbine arranged in an exhaust-gas discharge system and a compressor arranged in an intake system, comprises during a load increase Δpme, retarding ignition timing away from an ignition time αZ,opt optimized with regard to efficiency and beyond an ignition time αZ,knock, where αZ,knock is an earliest ignition time to avoid knocking combustion.
In this way, in the event of a load increase or step Δpme, the ignition time αZ is shifted in the late direction proceeding from an ignition time αZ,opt optimized with regard to efficiency. Contrary to the conventional approach, however, the ignition time αZ is shifted further in the late direction than is necessary to avoid knocking combustion. Whereas it is was case in previous systems that even during the course of a load step, the principle of as early as possible an ignition is adhered to, that is to say ignition takes place as early as possible, in order to ensure high efficiency, it is the case according to the disclosure that the ignition time αZ is shifted in the late direction beyond an ignition time αZ,knock. Here, αZ,knock denotes the earliest ignition time at which no knocking combustion is observed.
The shift of the ignition time in the late direction leads to a shift of the combustion or of the focal point of the combustion in the late direction, that is to say in the direction of the expansion phase.
In this way, the energy released during a first portion of a response to a step load increase of the course of the combustion by the exothermic chemical conversion of the fuel is utilized proportionally less in the form of expansion work, that is to say for increasing the gas forces acting on the piston, and more for increasing the exhaust-gas enthalpy, that is to say for increasing the exhaust-gas temperature and the exhaust-gas pressure. Conversely, a second portion of the response is applied more for expansion work and correspondingly less for exhaust enthalpy. The transition between the first and second portions may be based on a size of the load step (later for larger step increases as compared to smaller step increases), a starting speed of the turbocharger (earlier for higher starting speeds of the turbocharger than for lower starting speeds), etc.
As a result, the hot exhaust gases in the exhaust-gas discharge system at the inlet into the turbine also have a higher exhaust-gas enthalpy which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature. The response behavior of the turbine, of the exhaust-gas turbocharger and of the supercharged internal combustion engine is improved in this way.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.